1. Field of the Invention
The present invention relates to a glass base material, single mode optical fiber, a method for manufacturing thereof, and a method for detecting a defect thereof. More particularly, the present invention relates to a glass base material, single mode optical fiber, a method for manufacturing thereof, and a method for determining a cause of defect thereof by which a part of an optical fiber that causes a transmission loss can be easily specified.
2. Description of the Related Art
As one of the methods to measure a transmission loss in an optical fiber, there is a cutback method. The cutback method inputs a light having a predetermined wavelength into one end of an optical fiber and measures a power of light that exits from the other end of the optical fiber. Next, an incident side of the end of the optical fiber is cut for substantially 2 m.
Then, a light having a predetermined wavelength is input into one end of an optical fiber, which is about 2 m in length, and a power of light that exits from the other end is measured again. A difference between the two powers of lights is calculated. The difference of the two powers of lights is a transmission loss occurring in the remaining optical fiber, the power of light that traveling through which is not measured.
The cutback method can accurately measure an average transmission loss for the whole length of the optical fiber. However, it is difficult to measure a distribution of a transmission loss in the longitudinal direction of the optical fiber by the cutback method. The cutback method has the following disadvantage. In a case where the transmission loss is high, the cutback method cannot obtain the information whether the transmission loss is high over the whole length of the optical fiber or the transmission loss is high only on a part of the optical fiber. Also, the cutback method cannot detect the location of the part having a high transmission loss in the optical fiber.
As another method for obtaining the information of transmission loss in the longitudinal direction of the optical fiber, there is an OTDR (Optical Time Domain Reflectometer) method. The OTDR method measures a transmission loss in the optical fiber by inputting a pulse light having a predetermined wavelength from one end of the optical fiber. The OTDR method then measures a Rayleigh-scattering-light, which is returned from a position of z of the optical fiber to one end of the optical fiber, to which the pulse light is input. The “z” is a distance from the incident end plane of the optical fiber. Hereinafter, the Rayleigh-scattering-light is referred to as a backscattering light.
The strength of the backscattering light P(λ, Z) is calculated by the following equation (1).P(λ,z)=P0·α(λ,z)·B(λ,z)exp{−2∫0zγ(x)dx}  (1)    P0: strength of a propagation light at an incident end (z=0)    α: Rayleigh-scattering-coefficient.    B: backscattering-light-collect-coefficient    γ: local transmission loss.
When the equation (1) is transformed using a logarithmic value and is expressed by a dB scale, the equation (1) is transformed into an equation (2).
                                                                        S                ⁡                                  (                                      λ                    ,                    z                                    )                                            =                            ⁢                              10                ⁢                                                                  ⁢                log                ⁢                                                                  ⁢                                                      P                    ⁡                                          (                                              λ                        ,                        z                                            )                                                                                                                                              =                            ⁢                                                5                  ⁢                                                                          ⁢                  log                  ⁢                                                                          ⁢                                                            P                      0                                        ·                                          α                      ⁡                                              (                        λ                        )                                                                                            +                                  5                  ⁢                                                                          ⁢                  log                  ⁢                                                                          ⁢                  B                  ⁢                                                                          ⁢                                      (                                          λ                      ,                      z                                        )                                                  -                                  10                  ⁢                                                                          ⁢                                      (                                          log                      ⁢                                                                                          ⁢                      e                                        )                                    ⁢                                                                          ⁢                                                            ∫                      0                      1                                        ⁢                                                                  γ                        ⁡                                                  (                          x                          )                                                                    ⁢                                              ⅆ                        x                                                                                                                                                    (        2        )            
As shown in FIG. (2), the backscattering-light-strength S(λ, z) changes according to the position “z” in the longitudinal direction of the optical fiber. Here, the Rayleigh scattering coefficient α is assumed to be substantially constant along the longitudinal direction of the optical fiber when the transmission loss in the longitudinal direction of the optical fiber is relatively stable.
FIG. 1 shows an example of a result of typical OTDR measurement. A part, where the backscattering-light-strength S(λ, z) simply decreases, indicates that the characteristic of the transmission loss is stable. The abrupt change in the inclination of the line around z=10,000 m indicates that the abrupt increase in the transmission loss occurs at the position z=10,000 m.
As a cause of the abrupt increase in the transmission loss, such as a macro bending loss, which occurs when the optical fiber is bent by stress applied on the optical fiber. The transmission loss may abruptly increase when there is a defect in the connection between two optical fibers. The region having a high transmission loss is not desirable for transmitting a light signal. It is necessary to re-lay or re-connect the optical fiber in the region having a high transmission loss.
It is difficult to accurately measure the transmission loss by the OTDR method if only one side of the backscattering light, which is input to one side of the end of the optical fiber and returned to this side of the end of the optical fiber, is measured. Hereinafter, the measurement of the transmission loss by the OTDR method is referred to as OTDR measurement.
As made clear from the equation (1), the factor, which influences the strength of the backscattering light, is not limited to the transmission loss γ(z). The fluctuation in the backscattering-light-collect-coefficient B(z) also influences the strength of the backscattering light. Thus, the waveform of the light that propagates through the optical fiber fluctuates when the backscattering-light-collect-coefficient B(z) fluctuates.
In order to measure a transmission loss accurately, the backscattering light is measured from both ends of the optical fiber. Therefore, one backscattering-light-strength S1(λ, z) is measured from one end of the optical fiber, and another backscattering-light-strength S2(λ, z−L) is measured from another end of the optical fiber. Thus, the values of the backscattering-light-strength S1(λ, z) and S2(λ, z−L) shown in the following equations (3) and (4) are obtained.S1(λ,z)=5 log P01·α(λ)+5 log B(λ,z)−10(log e)∫0zγ(x)dx  (3)S2(λ,z−L)=−5 log P02·α(λ)−5 log B(λ,z)+10(log e)∫0zγ(x)dx  (4)
Then, a value of D(z) is obtained by the following equation (5). As shown in equation (5), the factor of the backscattering-light-collect-coefficient B(z) is canceled out by adding the values of the backscattering-light-strengths S1(λ, z) and S2(λ, z−L). Therefore, only the component of the transmission loss γ remains in equation (5).
                                                                        D                ⁡                                  (                  z                  )                                            =                                                {                                                                                    S                        1                                            ⁡                                              (                                                  λ                          ,                          z                                                )                                                              +                                                                  S                        2                                            ⁡                                              (                                                  λ                          ,                                                      z                            -                            L                                                                          )                                                                              }                                /                2                                                                                        =                                                const                  .                                      -                    10                                                  ⁢                                  (                                      log                    ⁢                                                                                  ⁢                    e                                    )                                ⁢                                                                  ⁢                                                      ∫                    0                    z                                    ⁢                                                            γ                      ⁡                                              (                        z                        )                                                              ⁢                                          ⅆ                      x                                                                                                                              (        5        )            
If the optical fiber is in a condition where the optical fiber is wound around a bobbin right after the manufacture of the optical fiber, it is possible to perform OTDR measurement from both ends of the optical fiber. However, it is extremely difficult to perform OTDR measurement from both ends of the optical fiber if the optical fiber is formed into cable and is laid linearly for more than 10 kilometers.
Hence, in the actual construction site, it was difficult to specify the part, in which the transmission loss is occurred, by measuring the backscattering-light-strength S(λ, z) from only one end of the optical fiber. If the backscattering-light-strength S(λ, z) is measured from only one end, the part having a large backscattering-light-collect-coefficient B(z) may be mistakenly considered as the part in which the transmission loss is occurred.
Thus, if there is a part having a large backscattering-light-collect-coefficient B(z) in the optical fiber, this part may be mistakenly considered to have a high transmission loss even if the transmission loss is actually low in this part.